1. Field of the Invention
The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery using the electrode.
2. Related Art
The battery performance of lithium secondary batteries recently under intensive research and development, such as charge-discharge voltage, cycle life characteristics or storage characteristics, depends largely upon the types of the electrodes used. This has led to the attempts to better battery performance by improving electrode active materials.
The use of metallic lithium for the negative active material, although possible to construct a battery with high energy density per weight and volume, presents a problem that the lithium deposited on charge grows into dendrite which could cause internal short-circuiting.
Lithium secondary batteries are reported (Solid State Ionics, 113-115, p57 (1998)) which use an electrode consisting of aluminum, silicon, tin or the like that is electrochemically alloyed with lithium on charge.
We have found that among electrodes for a lithium secondary battery using tin as an active material, an electrode comprising a tin thin film deposited on a current collector such as a copper foil by electroplating or the like has a large discharge capacity and relatively good cycle characteristics. In such an electrode having a tin thin film as an active material, cycle properties are believed to be seriously affected by the reaction which is easy to take place between the surface of the thin film and an electrolyte solution.
It is the object of the present invention to provide an electrode for a lithium secondary battery using a thin film of the active material containing tin as a main component and being capable of suppressing the reaction between an electrolyte solution and the active material.
The present invention provides an electrode for a lithium secondary battery comprising a current collector composed of a metal incapable of alloy formation with lithium, a thin film formed by depositing on the current collector and composed of an active material containing tin as a main component, and a tin oxide layer formed on the surface of the thin film.
In the present invention, due to the presence of the tin oxide layer on the surface of the thin film, the reaction between the electrolyte and the active material can be suppressed and the initial charge-discharge efficiency can be improved. Further, it is supposed that due to the presence of the tin oxide layer on the surface of the thin film, an oxide coating film suitable for improving the charge-discharge cycle characteristics is formed by electrochemical oxidation of the electrolyte during initial charge. It is therefore supposed that such an oxide coating film suppresses the separation of the thin film from the current collector to provide the improvement of the charge-discharge cycle characteristics.
In the present invention, the tin oxide layer preferably has a concentration gradient that the concentration of the tin oxide gradually decreases toward the current collector. Further, the thickness of the tin oxide layer is preferably 10 nm or more.
The concentration gradient may be varied continuously or in step wise. Due to the presence of the concentration gradient, the reaction of the active material in the thin film surface with the electrolyte is more effectively suppressed, and the cracking of the thin film caused by charge and discharge is effectively controlled, thereby to prevent the separation of the thin film.
Further, the content of the oxide layer in the entire thin film is preferably in the range of the amount corresponding to the oxygen content of 0.3% or more by atomic ratio of oxygen element with respect to tin element in the entire thin film. It is also preferably corresponding to the oxygen content of 20% or less. Consequently, it is preferable that the tin oxide layer is formed on the surface of the thin film so that the oxygen content in the thin film is in a range from 0.3 to 20% by atomic ratio of oxygen element with respect to tin element. If the oxygen content is too low, the effect of the present invention to suppress reaction with the electrolyte may not be sufficiently achieved. On the contrary, if the oxygen content is too high, the charge-discharge efficiency may be decreased.
The thin film of the active material in the present invention is a thin film containing tin as a main component, and is not particularly restricted as far as it can store lithium by alloying and release lithium electrochemically. Examples of such a thin film are a tin film composed of substantially tin alone and an alloy thin film containing tin as a main component. The alloy thin film is exemplified by Snxe2x80x94Pb, Snxe2x80x94Co, and Snxe2x80x94In thin films.
The tin oxide layer in the present invention may be any oxide layer containing tin oxide as a main component. For example, in the case the thin film is composed of a tin alloy, the tin oxide layer may contain an oxide of alloying element other than tin.
The thin film in the present invention can be formed by vapor phase or liquid phase deposition. For example, the thin film can be formed by a thin-film forming method such as electrolytic plating, electroless plating, CVD, sputtering, vacuum evaporation, spraying or the like.
The tin oxide layer in the present invention can be formed by various methods. For example, the tin oxide layer can be formed by oxidizing the surface of the thin film. Further, as another method, the tin oxide layer can be formed by depositing the tin oxide layer on the surface of the thin film.
The method for oxidizing the surface of the thin film is exemplified by a method of heat treatment of the thin film in the presence of oxygen. If the tin oxide layer is formed by oxidizing the surface of the thin film, the concentration gradient gradually decreasing the concentration of tin oxide toward the current collector can be easily provided.
In the method for forming the tin oxide layer by heat treatment in the presence of oxygen, the heat treatment temperature is preferably about 50 to 105% of the melting point of tin. Since the melting point of tin is 232xc2x0 C., it is preferably within a range from 116xc2x0 C. to 243xc2x0 C., and taking the reactivity of oxidation into consideration, it is further preferably within a range from 160xc2x0 C. to 240xc2x0 C.
Examples of the method for depositing the tin oxide layer are sputtering, vacuum evaporation, MOCVD, composite plating, and the like. After deposition of the thin film, the tin oxide layer is deposited by these methods to form the thin film having the tin oxide layer on the surface thereof.
In the case of forming the tin oxide layer by the sputtering method, for example, the tin oxide layer can be deposited using tin metal as a target and argon gas containing oxygen as the atmospheric gas. In this case, the oxygen concentration can be controlled by the flow rate of oxygen so as to control the concentration of tin oxide in the thickness direction of the thin film. For example, the oxygen gas flow rate is increased along with the deposition of thin film, the tin oxide concentration is decreased gradually toward the current collector. In other words, the concentration gradient with the tin oxide concentration gradually increasing toward the surface can be formed.
After the thin film of the active material is formed by the sputtering method, the tin oxide layer can be successively formed by adding oxygen gas to the atmospheric gas in the sputtering method. Consequently, the formation of the tin oxide layer by the sputtering method is especially useful in the case of forming the thin film by the sputtering method.
Formation of the tin oxide layer by the vacuum evaporation method can be carried out using tin oxide as an evaporation source.
In the case of the MOCVD method, the tin oxide layer can be formed by CVD using an organic metal such as an organic tin compound.
In the case of the composite plating method, the tin oxide layer can be formed by using a plating bath in which fine particles of tin oxide is dispersed and depositing tin oxide simultaneously with deposition of tin metal.
The tin oxide in the tin oxide layer may be single or a composite oxide containing tin oxides with different valence. The valence of tin in the tin oxide is preferable to be low since the initial charge-discharge efficiency is increased in such a case. Consequently, the tin oxide layer is preferable to contain SnO as a main component.
The crystallinity of tin oxide in the tin oxide layer is not specifically restricted and may be amorphous.
In the present invention, it is preferable that a mixed phase, in which the thin film component and the current collector component are mixed, is formed in the interface between the thin film and the current collector. Such a mixed phase can be formed by heat treating the thin film formed on the current collector, for example. Accordingly, the mixed phase may be formed simultaneously when the surface of the thin film is heat treated in the presence of oxygen for forming the oxide layer. The mixed phase may be composed of an intermetallic compound or solid solution of the thin film component and the current collector component. In the case Cu is the current collector component, an intermetallic compound of Sn and Cu may be formed in the mixed phase and a solid solution of Sn and Cu may be formed. Whether the mixed phase is composed of the intermetallic compound or the solid solution is determined by the thin film component, the current collector component, the composition ratio thereof, the mixed-phase forming conditions or the like.
In the present invention, the current collector is made of a metal incapable of alloy formation with lithium. The metal incapable of alloy formation with lithium is a metal which does not form a solid solution or an intermetallic compound with lithium (Li). Practically, a metal having no alloying state with Li in a binary phase diagram. Examples of the metal incapable of alloy formation with lithium include copper (Cu), iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), tungsten (W) and tantalum (Ta). Among these metals, copper is especially preferable to be used. The current collector is preferably thin and its thickness is preferably 50 xcexcm or less. From such a viewpoint, a metal foil, especially a copper foil is preferable to be used as the current collector.
It is preferable for the surface of the current collector to be rough. The rough surface improves the adhesion between the thin film and the current collector and suppresses the separation of the thin film from the current collector during charge and discharge. More specifically, the surface roughness Ra of the current collector is preferably about 0.01 to 2 xcexcm. The surface roughness Ra, which is specified by Japanese Industrial Standards (JIS B 0601-1994), can be measured by a surface roughness gage, for example. In the case a copper foil is used as the current collector, an electrolytic copper foil, which is a copper foil with a large surface roughness Ra, is preferable to be used.
The lithium secondary battery of the present invention comprises a negative electrode composed of the electrode of the present invention described above, a positive electrode, and a nonaqueous electrolyte.
An electrolyte solvent for use in the lithium secondary battery of the present invention is not particularly limited in type but can be illustrated by a mixed solvent which contains cyclic carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate, or vinylene carbonate and also contains chain carbonate such as dimethyl carbonate, methyl ethyl carbonate or diethyl carbonate. Also applicable is a mixed solvent of the above-listed cyclic carbonate and an ether solvent such as 1,2-dimethoxyethane or 1,2-diethoxyethane or a chain ester such as xcex3-butyrolactone, sulfolane or methyl acetate. Illustrative electrolyte solutes are LiPF6, LiBF4, LiCF3SO3, LiN (CF3SO2)2, LiN (C2F5SO2)2, LiN (CF3SO2)(C4F9SO2), LiC (CF3SO2)3, LiC (C2F5SO2)3, LiAsF6, LiClO4, Li2B10Cl10, Li2B12Cl12 and mixtures thereof. Other applicable electrolytes include a gelled polymer electrolyte comprised of an electrolyte solution impregnated into a polymer electrolyte such as polyethylene oxide, polyacrylonitrile or polyvinylidene fluoride, and inorganic solid electrolytes such as LiI and Li3N, for example. The electrolyte for the lithium secondary battery of the present invention can be used without limitation, so long as an Li compound as its solute that imparts an ionic conductivity, as well as its solvent that dissolves and retains the Li compound, remain undecomposed at voltages during charge, discharge and storage of the battery.
Examples of positive active materials for the present invention include lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, LiCo0.5Ni0.5O2 and LiNi0.7Co0.2Mn0.1O2; lithium-free metal oxides such as MnO2; and the like. Other substances can also be used, without limitation, if they are capable of electrochemical insersion and release of lithium.